Tone inhibitor circuit for plural tone receiver



May 27, 1969 w. J. COLE ET A1.

TONE INHIBITOR CIRCUIT FOR PLURAL TONE RECEIVER Filed March 7,

AHys.

United States Patent O 3,447,133 TONE INHIBITOR CIRCUIT FOR PLURAL TONERECEIVER William J. Cole, Arlington Heights, and Fred J. Conforti,Wooddale. Ill., assignors to Motorola, Inc., Franklin Park, Ill., acorporation of Illinois Filed Mar. 7, 1966, Ser. No. 532,342 Int. Cl.H04q 3/00 U.S. Cl. 340-171 9 Claims The present invention relatesgenerally to decoder circuits and more particularly to .a tone decodercircuit responsive to calling tone signals transmitted in apredetermined sequence to produce an alterting tone at a loudspeaker orthe like.

In the past various types of circuits have been employed to respond onlyto tones transmitted in a particular coded sequence to energize analerting device such as a loudspeaker. Among these circuits are thosewhich employ vibrating reeds in a mechanical filter arrangement as theinput frequency selective device. The reed vibrates strongly uponreception of signals .at the resonant frequency of the reed, thusproviding electromechanical coupling between input and output coils ofthe filter. Thus, the reed reconstitutes the input paging tone.

These prior art circuits have not proved entirely satisfactory in thatthey often produce false outputs from repetitive mechanical shocksduring decoder standby periods and in response to reed shocking incombination with tone signals forming part but not all of the completetone sequence to which the paging circuit is matched.

It is an object of the present invention to provide an improved tonedecoder or pager circuit that does not respond to mechanical shocks andgive false alerting tones.

It is another object of the invention to provide a novel tone decodercircuit that operates to insure that only tone signals received in apredetermined time sequence produce an alerting tone at the decodercircuit output.

A feature of the invention is the provision of first and second tonereceiving channels including frequency selective means in each channelfor passing tones arriving in a predetermined sequence, with an alertingdevice connected in the output of the second channel and adapted to beenergized by the final tone in the coded sequence. A timing circuit isconnected between the two channels and enables the final tone toenergize the alerting device only within a particular time durationafter the termination of Ia first tone signal.

Another feature of the invention is the provision of a decoder of thetype described including ia clamping circuit connected between theoutput of the timing circuit and a triggering circuit in the secondchannel for clamping the triggering circuit at ground potential at alltimes with the exception of a particular time period during theswitching of the timing circuit by a first tone signal, thereby enablingthe second channel to pass a nal tone signal to an alerting device onlywithin the time period.

Another feature of the invention is the provision of a shock inhibitorcircuit connected between the input of the first receiving channel andthe timing circuit and circut means connecting the first frequencyselective device to the clamping circuit and connecting the secondfrequency selective device to the shock inhibitor circuit to totallydisable both first and second channels by simultaneous signals resultingfrom accidental shock of the vibrating reeds of both the first andsecond frequency selective elements.

The invention to be described is illustrated in the accompanyingdrawings wherein:

FIG. l is a block diagram of the two channel decoder circuit; and

FIG. 2 is Ia detailed schematic diagram of the decoder lCe circuitshowing the corresponding functional blocks of FIG. 1 enclosed by dottedlines.

Briefly described, the tone decoder according to the present inventionincludes, as previously mentioned, first and second tone receivingchannels including, respectively, first and second frequency selectivemeans for receiving tones arriving in a predetermined sequence. Each ofthe channels includes a trigger circuit adapted to be triggered by thefirst and second tones respectively applied to the two channels, and atone alerting device is connected in the output of the second channeland adapted to be energized when the triggering circuit in the secondchannel is triggered by a second tone signal. A timing circuit isconnected in the output of the first channel and includes a monostablemultivibrator adapted to be switched by a first tone signal. A clampingcircuit is connected between the monostable multivibrator and thetrigger circuit in the second receiving channel, and when the decoder isin a standby position, the clamping circuit clamps the second channeltrigger circuit to ground potential. Upon the application of a firsttone signal and a subsequent monostable switching action of themultivibrator, the clamping circuit is momentarily cut off to enable thesecond channel trigger circuit only for a particular time duration. Ifthe second tone signal is present during the particular time duration,the second channel trigger circuit will be triggered and energize a toneoscillator in the output of the second channel to provide an alertingtone signal for driving a loudspeaker or the like to alert the personpaged.

Referring now in detail to FIGS. l and 2 of the drawing, there is showna pair of input reed circuits 4 and 5, each including input and outputcoils for coupling the reed circuits to a reed driver (not shown). Eachof these reed circuits is activated by a particular tone frequency andthe tones must arrive in a proper sequence to produce an alerting toneat the speaker 15, In one paging system having a 5 second call rate, afirst tone is transmitted for 1 second, followed Within 300 millisecondsby a second tone transmitted for 3 seconds. This example is given onlyby way of illustration, however, and each paging circuit performs aselection and timing function that is matched to a particular code.

When the initial tone is transmitted, the first tone decoding circuitsconnected to the output of the reed circuit 4 begin a timing action thatenables a subsequent tone signal appearing in the output of the reedcircuit 5 to energize the alert tone oscillator 14 provided that thesecond tone arrives at the reed circuit 5 within a particular timeduration as will -be explained later.

The resonant reeds used in circuits 4 and 5 are of the two-coil, singletine type. If a tone applied to the input coil is equal in frequency tothat of the mechanical resonance of the reed, the reed vibratesstrongly, acting as an electromechanical coupling device between inputand output coils. Therefore, each reed can be considered to be a verynarrow bandpass filter which passes only the desired paging tone.

The input reed circuits 4 and 5 are driven by the output of adiscriminator (not shown) which is amplified by a reed driver (notshown) to provide signals having an amplitude sufficient to activate thereed circuits. A first tone is coupled through the first reed circuit 4to the rst tone amplifier circuit 6. Here the first tone is amplified bytransistor 21 and applied to the first tone trigger circuit 8 includingtransistors 22 and 23. In the standby condition, both transistors 22 and23 are cut off with the co1- lector of transistor 22 at some positivevoltage and the collector of transistor 23 essentially at ground. When afirst tone is present, the positive swing of the signal biasestransistor 22 into conduction and the resultant drop of its collectorvoltage is a forward bias to transistor 23. This forward bias causestransistor 23 to conduct and its collector voltage goes positive. Thepositive voltage at the collector of transistor 23 applies additionalbias to the base of transistor 22 and the transistors 22 and 23 aredriven on through regenerative coupling.

The collector of transistor 23, which is the output of the first tonetrigger circuit 8, remains at some positive voltage during the presenceof the first tone at the input of reed circuit 4 and for a period afterthe termination of the first tone until the resonant reed in circuit 4stops vibrating. The time during which the first tone is actuallypresent will be referred to hereinafter as time t1 and the timebeginning with the termination of the initial tone and ending with thetermination of the reed vibration will be referred to as time t2. At theend of time t2 the first tone trigger circuit 8 returns to its standbycondition with the output `at Zero volts.

The positive pulse output of the first tone trigger circuit 8 is coupledthrough transistor 24 in the shock inhibitor circuit 10 to the collectorof transistor 25 in the multivibrator time delay circuit 11. Duringnormal operation, the shock inhibitor can be considered a series gate orswitch in the closed position, permitting a tone signal to pass. Itsoperation to prevent false paging from mechanical shocks of the reeds inthe input circuits 4 and 5 is explained later in the specification.

Transistors 25 and 26 form the multivibrator time delay circuit 11, andtransistor 25 is cut off and transistor 26 is conducting in the standbycondition. A tone gate or clamp circuit 13, including transistor 27, isconnected through resistor 41 and diodes 42 and 43 respectively to theoutputs of transistors 25 and 26. This gate or clamp 13 is alsosaturated when the circuit is in a standby condition. Before the initialtone is received, the positive collector potential of transistor 25 isresistively coupled through resistor 41 to the base of transistor 27 toprovide a forward bias therefor and allow transistor 27 to conduct tosaturation. The multivibrator time delay circuit 11 and the clampingcircuit 13 are represented as a single timing circuit 16 in the blockdiagram of FIG. l. As will be described hereinafter, the timing circuit16 serves both to (l) provide the proper timing function for channel twowhen first and second tones are received in a predetermined sequence and(2) provide inhibiting action for channel two when a first tone isreceived by coupling the positive pulse output from first tone trigger 8through line 18 and diode 46 to the input of the clamp 13.

The path of conduction for clamp 13 when the decoder circuit is instandby condition is through the transistor 27 collector and resistor 47in the base of the second tone trigger transistor 29 to the positiveterminal 52 of a positive voltage supply. The collector of 27 isessentially at zero volts and clamps the base of the second tone triggertransistor 29 sufficiently below cutoff to prevent activation by anytone signal that might be present at the input reed circuit 5 in channel2 of the decoder. Additionally, the saturated clamp 13 presents a verylow impedance to and virtually shorts out any signal coupled to the baseof transistor 29. Therefore, the alerting tone oscillator 14 cannot beactivated solely by a tone in channel 2.

During the positive pulse output from the first tone trigger which iscoupled through transistor 24 to the collector of transistor 25, thevoltage at the collector of 25 increases slightly. At the end of timet2, however, the collector of 25 tends to return to its initial voltageprior to the application of a first tone signal, and this decrease involtage generates a negative transition which is coupled throughcapacitor 44 to the base of transistor 26, cutting 26 olf. With 26cutoff, its collector rises to some positive potential which is coupledto the transistor clamp 27 to keep it conducting. This positivepotential at the Collector of 26 is also coupled back to the base oftransistor 25 via feedback resistor 33 and drives transistor 25 intoconduction for the first time. At this point the collector of 25approaches zero, dropping almost instantaneously from its positivevoltage during non-conduction. This change in potential is also coupledthrough capacitor 44 to the base of transistor 26 driving it far belowcutoff. As capacitor 44 discharges exponentially, the base voltage of 26begins to rise until it again reaches the turn on point for transistor26. When 26 begins to conduct, its negative going collector voltage iscoupled to the base of 25 tending to cut off transistor 25. However, astransistor 25 is driven to cutoff, its collector potential cannotimmediately rise to a voltage sufficiently positive to keep clamp 27conducting since capacitor 44 must be recharged. This is the first timethat either the collector of transistor 25 or the collector oftransistor 26 have not been able to apply a positive voltage throughdiodes 42 and 43 to the base of transistor 27 sufficient to keeptransistor 27 conducting. Thus, transistor 27 is unclamped until thecapacitor V44 has had time to charge and until the collector of 25 canagain reach a positive potential sufficient to drive transistor 27 intoconduction.

With the transistor 27 unclarnped, the second tone trigger 9 is enabled,and a positive tone signal at the base of unclamped trigger transistor29 will drive this trigger into conduction. When 29 conducts a negativepulse is produced at the collector thereof and is applied to themultivibrator 12, including transistors 30 and 31. These transistorsform a monostable multivibrator circuit similar to the time delaymultivibrator 11 in channel 1.

In the standby condition both transistors 30 and 31 are cutoff, thecollector of 30 being at some positive potential and the collector of 31being at substantially ground potential. When the second tone trigger 9applies a forward bias to transistor 31, transistor 31 is driven intoconduction and the resultant regeneration in multivibrator 12 drivesboth transistors 30 and 31 to saturation and applies a forward bias tothe base of transistor 32 in the tone oscillator audio output stage 14.Transistor 32 produces an alerting tone in the speaker 15 as long asthis forward bias remains.

The second channel multivibrator 12 returns to its initial state and thealerting tone at the loudspeaker 15 ceases when capacitor 48 becomesdischarged through resistor 49 or resistor 50, whichever is connected inthe base circuit of transistor 30. This alerting tone period may bevaried substantially by changing the jumper connection to resistors 49and 50. An operator may end the alerting tone sooner, however, bypushing the switch 53 to ground the base of transistor 30'.

The level of the alerting tone at the output of the loudspeaker 15 canbe decreased by adding resistance in the emitter circuit of transistor32 to sharply reduce transistor current.

When the pager is initially turned on, a strong steady alerting tone istemporarily produced to indicate that the battery voltage is sufficientfor pager operation. This is due to the fact that voltage transientsmomentarily trigger transistor 31 into conduction when a battery voltageis first applied, and regeneration through capacitor 48 sets the secondmultivibrator 12 into operation, just as if a paging signal werereceived.

The shock inhibitor circuit 10 takes advantage of the fact that bothreeds in circuits 4 and 5 vibrate when shocked, but only one reed at atime vibrates during normal paging signal reception. In order tounderstand the function of the shock inhibitor, consider the situationwhen both reeds are simultaneously shocked. Upon such condition, thetone in the first channel actuates the first tone trigger 8 and couplesan output signal via line 18 to the base of transistor clamp 27, keepingit saturated and disabling the second tone trigger 9 as previouslydescribed. At the same time, a tone in channel 2 is coupled from thetone amplifier 7 through a voltage doubler rectifier circuit 20 andapplied to the base of the transistor 24 in the shock inhibitor circuit10. This biases the shock inhibitor to cutoff and prevents a tone signalfrom entering the multivibrator time delay circuit 11 so that the gatingpulse is not generated thereby. Thus, both the first and second channelsare simultaneously blocked at the inhibitor 10 and trigger 9respectively when the reeds in the input reed circuits 4 and 5 arecaused to resonate due to accidental shocks. On the other hand, fornormal operation an initial tone in channel 1 is coupled to the firsttone trigger 8 and passed by transistor 24 in the shock inhibitorcircuit 10 to energize the multivibrator time delay circuit 11. When thefirst tone ends, a second tone received in channel 2 is coupled throughthe voltage doubler 20` to the base of transistor 24 and biases theshock inhibitor 10 to cutoff. This assures that the first multivibrator11 input signal ends promptly upon the reception of a second tone inchannel 2.

Thus, the combination of the shock inhibiting action in 'both channels 1and 2 and the unique time delay feature provided by channel 1 to enablechannel 2 only for a particular time duration after the termination of atone signal in channel 1 insures that the decoder circuit Will onlyproduce an alerting tone at speaker when a proper coded sequence of tonesignals is received.

We claim:

1. A tone decoder including in combination: first tone receiving channelmeans including first frequency selective means for passing a first tonesignal of a first predetermined frequency, second tone receiving channelmeans including second frequency selective means for passing a secondtone signal of a second predetermined frequency, alerting means coupledto said second tone receiving channel means and adapted to be energizedby said second tone signal, enabling means coupled between said firstand second channels and acting normally to disable said second tonereceiving channel means, said enabling means being responsive to saidfirst tone signal for enabling said second tone receiving channel mean-sto pass ysaid second tone signal from said second frequency selectivemeans to said alerting means only within a particular time durationafter the termination of said first tone signal at the input of saidfirst frequency selective means, and inhibiting means coupled to saidsecond receiving channel and t0 said enabling means and being responsiveto said second tone signal to prevent said first tone signal from beingcoupled to said enabling means.

2. The decoder according to claim 1 wherein said first and secondfrequency selective means includes: first and second electromechanicalfilters respectively, each of said first and second electromechanicalfilters having a reed and input and output coil-s coupled to said reedwhereby input tone signals at said input coils cause said reeds tovibrate strongly when said tone signals have a frequency equal to theresonant frequency of said reeds and to thereby produce output tonesignals in said coils which exceed a predetermined amplitude.

3. The decoder according to claim 1 wherein said enabling meansincludes: switching means coupled to said first tone receiving channelmeans and normally in a first conductive state, said switching meansbeing responsive to the termination of said first tone signal in saidfirst tone receiving channel means to change the conductive state ofsaid switching means to a second conductive state, said switching meansremaining in said second conductive state for a predetermined timeperiod, said switching means further `acting to change to said firstconductive state at the end of said predetermined time period and todevelop a control signal for a particular time duration, said enablingmeans further including clamping means coupling said switching means tosaid second tone receiving channel means and normally acting to disablethe same, said clamping means being responsive to said control signal toenable said second tone receiving channel means to pass said second tonesignal to said alerting means only within said particular time duration.

4. The decoder according to claim 3 wherein said switching meansincludes a monostable multivibrator comprising a pair of semiconductordevices adapted to be switched between said first and second conductivestates, a capacitor coupled between said semiconductor devices andadapted to charge and discharge during monostable switching action ofsaid multivibrator when a first tone signal is applied to one of saidsemiconductor devices, said decoder further including, circuit meanscoupling said capacitor to said clamping means and responsive to saidchange in said conductive state from said second to said firstconductive state to provide a voltage change at said clamping meansproportional to the charge time of said capacitor, said clamping meanschanging its conductive state during said voltage change to enable saidsecond receiving channel means to pass said second tone signal onlyuntil said capacitor is charged to a predetermined voltage.

5. A tone detector including in combination: a first tone receivingchannel including first frequency selective means for passing .a firsttone signal of a first predetermined frequency, a second tone receivingchannel including second frequency selective means for passing a secondtone signal of a second predetermined frequency, first and secondtrigger means coupled to said first and second frequency selective meansrespectively, said first trigger means being responsive to said firsttone signal to generate a first trigger signal, alerting means, circuitmeans coupling said second trigger means to said alerting means, saidalerting means being responsive to a second tone signal applied to saidsecond frequency selective means t0 develop an alerting tone, timingcircuit means, shock inhibiting means coupled to said second frequencyselective means and coupling said first trigger means to said timingcircuit means, said timing circuit means including clamping meanscoupled to said second trigger means for normally disabling the same,said timing circuit means being responsive to lsaid first trigger signalto generate a control signal for a particular time duration apredetermined time interval after the termination of the first triggersignal, said clamping means being responsive to said control signal toenable said second trigger means whereby said second tone signalreceived during said particular time duration actuates said alertingmeans, said shock inhibiting means being responsive to said second tonesignal to block the coupling of said first trigger signal to said timingcircuit means to thereby prevent the generation of said control signal.

6. The decoder according to claiml 5 wherein said timing circuitincludes: a monostable multivibrator circuit coupled to said clampingcircuit and normally in a first conductive state, said monostablemultivibrator being responsive to the termination of said first triggersignal to assume a second conductive state and to remain therein for apredetermined time interval, said multivibrator further acting to returnto said first conductive state at the end of said predetermined timeinterval and to generate said control signal for said particular timeduration upon said return to said first conductive state.

7. The decoder according to claim 6 wherein: said Second trigger meansincludes a first transistor having a base electrode coupled to saidsecond frequency selective means, and said clamping means includes asecond transistor coupled to said monostable multivibrator and normallybiased to a conductive state thereby, said second transistor beingcoupled to said base electrode of said first transistor, said secondtransistor in its conductive state acting to bias said first transistorto non-conduction and to bypass signals appearing on said base electrodeof said first transistor whereby said second trigger circuit isdisabled, said second transistor being responsive to said control signalto become non-conductive, said second transistor acting in itsnon-conductive state to bias said first means, said transistor meansnormally being in a conductive state, voltage doubler means couplingsaid second frequency selective means to said transistor means, saidvoltage doubler means and said transistor means being responsive to saidsecond tone signal to bias said transistor means to non-conductionwhereby said lirst trigger signal is prevented from actuating saidmonostable multivibrator means.

9. The decoder according to claim 5 wherein circuit means couples saidrst trigger means to said clamping 8 means for applying said lirsttrigger signal thereto, said clamping means being responsive to said rsttrigger signal to maintain said second trigger means in said disabledcondition.

References Cited UNITED STATES PATENTS 3,355,709 11/1967 Hanus 340-171JOHN W. CALDWELL, Primary Examiner. HAROLD T. PITTS, Assistant Examiner.

U.S. Cl. X.R. 340-164

1. A TONE DECODER INCLUDING AN COMBINATION: FIRST TONE RECEIVING CHANNELMEANS INCLUDING FIRST FREQUENCY SELECTIVE MEANS FOR PASSING A FIRST TONESIGNAL OF A FIRST PREDETERMINED FREQUENCY, SECOND TONE RECEIVING CHANNELMEANS INCLUDING SECOND FREQUENCY SELECTIVE MEANS FOR PASSING A SECONDTONE SIGNAL OF A SECOND PREDETERMINED FREQUENCY, ALERTING MEANS COUPLEDTO SAID SECOND TONE RECEIVING CHANNEL MEANS AND ADAPTED TO BE ENERGIZEDBY SAID SECOND TONE SIGNAL, ENABLING MEANS COUPLED BETWEEN SAID FIRSTAND SECOND CHANNELS AND ACTING NORMALLY TO DISABLE SAID SECOND TONERECEIVING CHANNEL MEANS, SAID ENABLING MEANS BEING RESPONSIVE TO SAIDFIRST TONE SIGNAL FOR ENABLING SAID SECOND TONE RECEIVING CHANNEL MEANS